Dipropylene glycol tert-butyl ether (DPTB) is a useful solvent for cleaners and coatings because of its excellent solvency power, low odor, good coalescing properties, and relatively low vapor pressure. DPTB can be made by reacting dipropylene glycol with isobutylene in the presence of an acidic catalyst. Crude DPTB made by this method contains unreacted dipropylene glycol and di-tert-butyl ethers derived from dipropylene glycol. DPTB is more volatile than dipropylene glycol, and the two can be separated successfully by ordinary distillation techniques. Di-tert-butyl ethers derived from dipropylene glycol, on the other hand, are not easily separated from DPTB because the compounds have similar boiling ranges.
To minimize the amount of di-tert-butyl ethers generated in the process, DPTB can be prepared using an excess of dipropylene glycol, so that the major separation problem is in isolating DPTB from unreacted dipropylene glycol. The level of di-tert-butyl ethers remaining in the DPTB product will then be quite small.
Unfortunately, di-tert-butyl ethers derived from dipropylene glycol are not miscible with water. Consequently, even a low concentration of diether impurities in DPTB significantly impairs water miscibility and makes aqueous mixtures of DPTB appear hazy. Removal of the di-tert-butyl ether impurities is needed to give a DPTB product having satisfactory water miscibility. However, because of the similarity in boiling points between DPTB and the di-tert-butyl ether impurities, purification by ordinary distillation is impracticable.
Extractive distillation is often used to separate compounds having similar boiling points. In this technique, an extracting agent is included in the distillation process to enhance the boiling point difference between the compounds of interest. For example, Berg et al. (U.S. Pat. No. 4,585,526) teach to separate o-xylene from m-xylene using propoxypropanol as an extractive agent. Although many extractive distillation processes are known, we are unaware of one for separating glycol monoethers from glycol diether impurities.
A way to recover DPTB from mixtures of DPTB and a minor proportion of di-tert-butyl ether impurities derived from dipropylene glycol is needed. Preferably, the process could enhance the purity of DPTB enough to overcome the water miscibility problems created by the presence of the diethers. A preferred process would be easy to practice, and could be coupled with the process for making DPTB from dipropylene glycol and isobutylene.